1. Field of the Invention
The present invention relates to a hydraulic lash adjuster used in valve gears of internal combustion engines.
2. Description of the Related Art
Japanese Patent Application Publication, JP-A-2004-278377, discloses a cylindrical bottomed body and a cylindrical bottomed plunger which is provided in the body so as to be lifted therein. A high-pressure chamber is defined by a lower end of the body and a bottom wall of the plunger. When the plunger is lifted downward, operating oil in the high-pressure chamber is adapted to leak through a gap between an inner periphery of the body and an outer periphery of the plunger.
The plunger has an upper end on which a proximal end of a rocker arm is placed. The proximal end of the rocker arm serves as a rocking fulcrum. The rocker arm has a free end that presses an upper end of a valve stem. The rocker arm is vertically rocked with rotation of a cam slid on an upper surface thereof. A valve is opened by upward movement of the rocker arm, whereas the valve is closed by downward movement of the rocker arm.
When a valve gear causes eccentric movement, the plunger is sometimes moved upward excessively over a normal range of upward and downward movement. Since the rocking fulcrum of the rocker arm is elevated in this case, a cam base is brought into sliding engagement with the rocker arm.
Conventional lash adjusters include a leak path through which operating oil in a high-pressure chamber is caused to leak with downward movement of the plunger. The leak path comprises a narrow gap between an outer periphery of the plunger and an inner periphery of the plunger body. The plunger needs to be quickly moved downward in order that the aforenoted drawback may be avoided. However, resistance of operating oil to flow through the narrow leak path is high. An elastic returning force of a valve spring biasing the valve in a closing direction is increased when the valve is opened, whereupon load the plunger receives from the rocker arm is also increased. Since the resistance of operating oil to flow through the leak path is large as described above, the plunger cannot be quickly moved downward even when having received such a large load as described above.
Increasing a dimensional difference between the outer diameter of the plunger and the inner diameter of the body has simply been considered as means for increasing the descending speed of the plunger, whereupon the sectional area of the leak path can be increased. Consequently, the resistance of operating oil to flow through the leak path can be reduced. However, the leak path also serves as means for attenuating the load the rocker arm applies to the plunger during normal operation of the valve gear and the lash adjuster, thereby suppressing the downward movement of the plunger. Accordingly, the leak path cannot simply be spread.